Curable silicone compositions are used as sealing materials or protective coating materials for optical semiconductor elements in optical semiconductor devices such as light emitting diodes (LEDs), or the like. As such a curable silicone composition, a curable silicone composition comprising: a linear organopolysiloxane having at least two silicon-bonded alkenyl groups and at least one silicon-bonded aryl group in a molecule; an organopolysiloxane having at least one silicon-bonded alkenyl groups, at least one silicon-bonded aryl group, and a siloxane unit represented by the formula: C6H5SiO3/2 in a molecule; an organopolysiloxane having at least two silicon-bonded hydrogen atoms in a molecule; and a hydrosilylation-reaction catalyst has been proposed (see Patent Document 1).
However, such a curable silicone composition has a problem in that, although a cured product having a high refractive index can be obtained, the elongation of the cured product is low. Because the cured product exhibits high gas permeability, when such a cured product is used in high brightness LEDs, which exhibit high light intensity and generate large amounts of heat, problems occur such as discoloration of a sealing material, and a reduction in brightness due to corrosion of silver plated on the LED substrate by corrosive gases.